[Openmcl-devel] high res timer for CCL

Gary Byers gb at clozure.com
Wed Dec 10 02:05:01 UTC 2008


Carbon functions for measuring wall-clock time are deprecated (and in some
cases they seem to be missing on x8664 Leopard.)

Supposedly, the most accurate and lowest-overhead means of measuring
elapsed time on MacOS is #_mach_absolute_time, which returns an unsigned
64-bit integer representing real time in "Mach Time Units".  Exactly
how many Mach Time Units there are in a second is CPU-dependent; on
a few Core 2 Duo systems that I tried this on, there seem to be exactly
1000000000 of them (e.g., a Mach Time Unit is a nanosecond.)  On an
iMac G5, a Mach Time Unit appears to be around 300 nanoseconds.

We can determine the ratio of Mach Time Units to nanoseconds by
calling #_mach_timebase_info, letting it fill in the fields of a
"mach_timebase_info" structure.  (Those fields are a 32-bit unsigned
numerator and a 32-bit unsigned denominator.)  We'd presumably want to
cache this info and only compute it the first time we need it in a
lisp session.

(defvar *mach-time-unit-ratio*
   (rlet ((info #>mach_timebase_info))
     (#_mach_timebase_info info)
     (/ (pref info #>mach_timebase_info.numer)
        (pref info #>mach_timebase_info.denom))))

(defun measure-elapsed-time (thunk)
   (let* ((start (#_mach_absolute_time)))
     (funcall thunk)
     (let* ((elapsed-mach-time-units (- (#_mach_absolute_time) start)))
       (values (round (* elapsed-mach-time-units *mach-time-unit-ratio*))))))

? (measure-elapsed-time (lambda () (sleep 1)))
1000051891

which is probably pretty accurate (e.g, we sleep for 1000000000
nanoseconds, and it's another 50 or more microseconds before we
actually get scheduled again.)  You might see more lor less latency
depending on what else was runnable and how many CPU cores there are
to run it on;  (It's really neat that this has a resolution of a
nanosecond, but it can be very difficult to consistently get results
whose accuracy is particularly close to their precision.)

On x86, #_mach_absolute_time seems to be implemented in terms of the
rdtsc instruction, which returns the value of an on-chip counter that
increments on every clock cycle (e.g, somewhere around twice a nanosecond
on a modern x86 CPU).  Each CPU core has its own time-stamp counter,
OSX seems to do a good job of getting keeping and keeping them in
synch, but other OSes may not.  On many consumer machines, the CPU
frequency can be scaled down (to conserve poweer/generate less heat)
when system software decides (by some measure) that the system is
"idle" and scaled back up after it notices that it has gotten "busy",
so it can be hard to map from TSC values (cycles) to real time units
unless the mapping's done by code that's aware of these issues.

The old, deprecated Carbon functions (those that still exist) are
implemented in terms of #_mach_absolute_time.  Many of these
functions conceptually "return structures", which in C means that
the caller and callee conspire somehow to side-effect an instance
of the structure type in the caller.  In C this is written to look
like a regular assignment:

      AbsoluteTime start;

      start = UpTime();

but what goes on behind-the-scenes to arrange that 'start' is
side-effected by the call depends on the platform (and sometimes
on the size and type of the structure and its fields.)

It's supposed to be the case that structure assignment/return is
handled in CCL's FFI by passing a pointer to the structure as
a first argument in the call; that -should- cause the appropriate
behind-the-scenes activity to take place to maintain the illusion
of (and have the effect of) assignment:

   (rlet ((start #>AbsoluteTime))
     (_#UpTime start)
     ...)

Back in the day (on the 32-bit PPC, mostly), that was literally the
extent of the behind-the-scenes activity: the caller would pass an
invisible first arg - a pointer to a structure - to the callee, and
the callee would fill in that structure's fields.  The interface 
translator would silently translate "function returning structure" to
"function that accepted structure pointer as extra first arg and returned
:void".

Technically, I don't think that that was always correct for PPC32
(structures that contained a single 32-bit word may have been handled
differently, though the details are a bit blurry ...)  On PPC64 and
x86-64 platforms (and on Dawin on x86-32), the conventions are more
complicated: sometimes, the behind-the-scenes activity involved
returning a structure's fields or some composite words in some 
combination of general-purpose and floating-point registers.  The
FFI translator was changed to make structure return explicit, and
the FFI itself was changed to use platform-specific translation
rules for some parts of foreign-function call forms and callback
definitions.  We want to hide all of that gunk, and use syntax
like that in the #_UpTime call above regardless of what happens
behand-the-scenes/under-the-hood, but the FFI can only arange for
that stuff to happen if we know that the function being called
"returns a structure".

Unfortunately, that change to the FFI translator was controlled by
a DEFVAR, and at some point the initial value of that DEFVAR was
changed to revert back to the old PPC32 behavior.  As of a couple
of days ago, most of the interfaces distributed with CCL on all of
the platforms it runs on were missing infomation about explicit
structure return.  (This information was present on ObjC method
definitions, and structure-return conventions are most heavily
used on Darwin and there mostly in Carbon/Cocoa/CoreFoundation.)

I've been updating the interfaces to ensure that information about
structure returns is present when it should be over the last few
days.  If you update from svn, you should get a set of interfaces
where code like the above would work (if you have the right libraries
loaded - as you would in the IDE - and are using the right set of
interfaces, which might depend on how the IDE was built.  (For reasons
that seemed to make sense a year or so ago, the IDE is sometimes built
with a copy of the interfaces built into the .app bundle; that may
make them easier to find, but may also make them harder to update via
svn.)

If the last few paragraphs don't make sense, a shorter version is
that you can get the code fragment above to do the right thing
by avoiding #_:

   (rlet ((start #>AbsoluteTime))
     (external-call "UpTime" start #>AbsoluteTime)
     (let* ((mach-absolute-time (#_mach_absolute_time)))
       (format t "~&   uptime is ~d,~&mach time is ~d"
          (dpb (pref start #>AbsoluteTime.hi)
               (byte 32 32)
               (pref start #>AbsoluteTime.lo))
          mach-absolute-time)))

The values printed by this code snippet will differ by a few ms (if
we could call UpTime and mach_asolute_time at exactly the same time,
we'd get exactly the same answer.  I'd personally prefer to just
use the underlying #_mach_absolute_time directly and forget about
the deprecated Carbon stuff, but until it totally disappears it's
still possible to party like it's 1999.





On Mon, 8 Dec 2008, Alexander Repenning wrote:

> This new version of the high resolution timer makes it simple to
> measure the time spent in s-expressions down to the microsecond
> level.  This version is integrated into CCL editor. Select an
> expression and get the time it takes  to run. Quite precise actually.
>
> http://www.agentsheets.com/lisp/Clozure_CL/us%20Timer.lisp
>
> The only shame is that there would be an even better time function,
> good old upTime, with nanosecond resolution. Here is some code:
>
> #include <CoreServices/CoreServices.h>
>
> int main(){
>     AbsoluteTime start = UpTime();
>     //Do stuff
>     AbsoluteTime end = UpTime();
>
>     Nanoseconds diffNS = AbsoluteDeltaToNanoseconds(end,start);
>     long nanoseconds = UnsignedWideToUInt64(diffNS);
>
>
> How would one translate this into CCL  code?
>
> Alex
>
>
>
>
>
> Prof. Alexander Repenning
>
> University of Colorado
> Computer Science Department
> Boulder, CO 80309-430
>
> vCard: http://www.cs.colorado.edu/~ralex/AlexanderRepenning.vcf
>
>
> _______________________________________________
> Openmcl-devel mailing list
> Openmcl-devel at clozure.com
> http://clozure.com/mailman/listinfo/openmcl-devel
>
>



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